Multivariable transmitters are known in industrial control and measurement for use in measuring process variables. A typical multivariable transmitter has a number of sensors from which process data is gathered, and may include a processor for calculating a physical process parameter based upon the gathered process data. For example, a multivariable transmitter for measuring flow rates in a pipe may include a temperature sensor and one or more pressure sensors. Data from these sensors, along with other physical parameters, may be used to calculate a flow rate through the pipe on an ongoing basis.
One example of a multivariable transmitter is described, for example in U.S. Pat. No. 5,495,769 to Broden, et al. (xe2x80x9cBrodenxe2x80x9d). Broden discloses a multivariable transmitter that calculates flow through a pipe using processes measurements taken from at least three sensors, including a temperature sensor, a differential pressure sensor and an absolute pressure sensor. Broden discusses a need for reducing power consumption in field mounted multivariable measurement transmitters, and presents a system using simplified flow calculations that reduce processing demands on the transmitter. As a significant disadvantage, the system described in Broden may nonetheless use substantial power to excite sensors when taking process measurements. However, industrial standards such as those promulgated by the International Electrotechnical Commission (xe2x80x9cIECxe2x80x9d) may specify a current for field instrumentation of less than four milliamps, including all processors and other circuitry, along with current used to excite any sensors.
There remains a need for a multivariable transmitter that conserves the use of power to excite sensors. There also remains a need for an accurate, low-cost calibration technique that may be used with ratiometric measurements typical of bridge-type sensors found in multivariable transmitters.
The systems described herein include a multivariable transmitter with one or more bridge sensors for measuring absolute pressure, differential pressure, and temperature of a process fluid in a pipe. In order to reduce current draw, the sensors are excited one at a time, each during an on period of a drive signal. Each sensor output is captured by a gated integrator that is coupled to the sensor output within the on period. The gated integrator stores a value representative of the sensor output on a node that may then be sampled by an analog-to-digital converter. Using this approach, the analog-to-digital converter may sample at a frequency independent of the frequency of the sensor drive signal. For example, the analog-to-digital converter may sample at a frequency that is less than one-half the frequency of the on period, in order to avoid certain artifacts of the digitization process. The systems described herein also include a technique for ratiometric calibration of a data acquisition system that, independent of true voltages for sensor measurements, can reproduce highly accurate results from ratiometric sensors such as bridges.
A method for process measurement disclosed herein includes: providing a power source; selectively coupling the power source through a process measurement transducer with a first frequency, the process measurement transducer having an output; measuring the output of the process measurement transducer to provide a measured output; sampling the measured output at a second frequency that is independent of the first frequency to provide a sampled value; and providing a digital representation of the sampled value.
In another aspect, a method for process measurement disclosed herein includes: providing a drive signal, the drive signal having an on period at a first frequency; selectively driving one of a plurality of process measurement transducers in response to the drive signal; capturing the output of the one of the plurality of process measurement transducers at the first frequency; and digitally sampling the measured output at a second frequency that is independent of the first frequency.
In another aspect, a method for process measurement disclosed herein includes: providing a control signal, the control signal having one or more on periods and a first frequency at which the one or more on periods are repeated; driving a process measurement transducer with a power source in response to the control signal, the process measurement transducer having an output; capturing the output of the process measurement transducer within the on period of the control signal to provide a captured output; sampling the captured output at a second frequency that is independent of the first frequency to provide a sampled value; and providing a digital representation of the sampled value.
Capturing the output may include capturing the output on a node of a gated integrator. The gated integrator may include a low pass filter. Sampling the captured output may further include sampling the captured output with an analog-to-digital converter. The analog-to-digital convertemay be a sigma-delta analog-to-digital converter. The first frequency may be greater than the second frequency. The first frequency may be at least twice as high as the second frequency. The process measurement transducer may include a bridge having a differential output. The process measurement transducer may include at least one of an absolute pressure sensor, a differential pressure sensor, and a temperature sensor. The control signal may control operation of a switch, the switch coupling the power source to ground through the process measurement transducer. The methods above may further include applying the digital representation of the sampled value to calculate a process variable. The process variable may be a volume flow rate. The method may further include driving a plurality of process measurement transducers from the power source by selectively coupling the transducers to ground with a plurality of control signals, each of the control signals having an on period that does not coincide with the on periods of the other control signals.
A multivariable transmitter for measuring a process variable as disclosed herein may include: a power source; a driver, the driver providing a control signal having an on period and a first frequency at which the on period is repeated; a process measurement transducer selectively coupled between the power source and a ground in response to the control signal, the process measurement transducer having an output indicative of a process measurement; an integrator selectively coupled to the output of the process measurement transducer within the on period of the control signal, the integrator capturing the output of the process measurement transducer on a node of the integrator within the on period of the control signal; a digital sampler, the digital sampler acquiring a sampled value of the node of the integrator at a second frequency that is independent of the first frequency, and the digital sampler providing a digital representation of the sampled value; and a processor that receives the sampled value and calculates a process variable using the sampled value.
The process measurement may include at least one of an absolute pressure, a differential pressure, and a temperature of a process fluid. The process variable may be a flow rate of a process fluid. The processor may transmit at least one of the process variable or the process measurement to an external system.
In another aspect, a system for measuring a process variable as disclosed herein includes: a driver, the driver providing control signals having an on period and a first frequency at which the on period is repeated; a process measurement transducer driven with an excitation in response to the control signal, the process measurement transducer having an output indicative of a process variable; a gated integrator selectively coupled to the output of the process measurement transducer within the on period of the control signal, the gated integrator capturing the output of the process measurement transducer on a node of the gated integrator within the on period of the control signal; a digital sampler, the digital sampler acquiring a sampled value of the node of the gated integrator at a second frequency that is independent of the first frequency, and the digital sampler providing a digital representation of the sampled value; and a processor that receives the sampled value and calculates a process variable using the sampled value.
The driver may include a microcontroller. The process measurement transducer may include a bridge transducer having a differential output pair. The digital sampler may include a sigma-delta analog-to-digital converter.
In another aspect, a method for calibrating a device that has a plurality of selectable gains for use with ratiometric process measurements may include: selecting a first gain for a device; zeroing the device at the first gain to remove a first offset; measuring a fixed voltage with the device at the first gain to obtain a first measurement; selecting a second gain for the device; zeroing the device at the second gain to remove a second offset; measuring the fixed voltage with the device at the second gain to obtain a second measurement; calculating a ratiometric calibration constant that is proprotional to a ratio of the first measurement to the second measurement; and applying the ratiometric calibration constant to a process measurement that includes a ratio of a process measurement taken at the first gain and a process measurement taken at the second gain.
The method may include performing an initial calibration of the device using a known reference voltage. The method may include repeating measuring the fixed voltage with the device at the first gain to obtain an first average measurement; repeating measuring the fixed voltage with the device at the second gain to obtain a second average measurement; and calculating a ratiometric calibration constant using the first average measurement and the second average measurement. The method may include calculating a plurality of ratiometric calibration constants for a plurality of gains.